Grenoble ‘dairy cow’ helps studying nuclear properties of chemical element 100 in Mainz

Where does the periodic table of chemical elements end and which processes lead to the existence of heavy elements? An international research team reports on experiments performed at the GSI/FAIR accelerator facility and at Johannes Gutenberg University (JGU) in Mainz to come closer to an answer to such questions. They gained insight into the structure of atomic nuclei of fermium (element 100) with different numbers of neutrons. The results were now published in the scientific journal Nature.

“Using a laser-based method, we investigated fermium atomic nuclei, which possess 100 protons, and between 145 and 157 neutrons. Specifically, we studied the influence of quantum mechanical effects on the size of their atomic nuclei,” explains Sebastian Raeder, the head of the experiment at GSI/FAIR.

For these measurements, an international collaboration of 27 institutes from 7 countries examined fermium isotopes with lifetimes ranging from a few seconds to a hundred days. This was enabled using different methods for producing the fermium isotopes. The short-lived isotopes were produced (by fusion reactions) at the GSI/FAIR accelerator facility. The neutron-rich, long-lived fermium isotopes (^255,257Fm) were produced in picogram amounts by irradiations in high-flux research reactors. A first irradiation in the HFIR (High Flux Isotope Reactor) at Oak Ridge National Laboratory, TN, USA produced isotopes up to ²⁵⁷Fm. From this mixture of produced elements, radiochemists at Mainz University extracted the neighbouring element einsteinium (element 99), that was then irradiated in the high flux reactor of Institut Laue-Langevin (ILL) in Grenoble, France. The so produced ²⁵⁵Es, with 9‑months half-life, keeps decaying to ²⁵⁵Fm, with only 20 h half-life. The latter was repeatedly radio-chemically extracted at Mainz University (and used for laser spectroscopy). Radiochemists call this process “milking” a fermium cow.

“Interestingly these elements’ names are well-deserved eponyms for the production process: the famous E=mc² relation by Albert Einstein is the foundation of energy generation in nuclear reactors, while Enrico Fermi is the creator of the first man-made nuclear reactor and fermium is the heaviest element directly reachable by reactor irradiations,” points out Ulli Köster, ILL scientist and one of the authors of the publication. “This study demonstrates impressively the synergy of different production methods: accelerators are best suited to produce neutron-deficient isotopes (less neutrons than usual) while research reactors are best suited to produce neutron-rich isotopes. Extending such studies to superheavy elements often requires a combination of both techniques, namely accelerator-irradiation of reactor-produced targets,” Köster adds.

Using forefront laser spectroscopy techniques, subtle changes in the atomic structure can be analysed, which in turn provide information about nuclear properties such as the nuclear charge radius, i.e. the distribution of protons in the atomic nucleus. Laser light of a suitable wavelength lifts an electron in the fermium atom to a higher-lying orbital, and then removes it from the atom altogether, forming a fermium ion, which can be detected efficiently. The exact energy required for this stepwise ion-formation process varies with neutron number. This small change in excitation energy was measured to obtain information about the change in charge radius of the atomic nuclei.

The investigations provided insight into the changes of the nuclear charge radius in fermium isotopes across the neutron number 152 and showed a steady, uniform increase. The comparison of the experimental data with various calculations performed by international collaboration partners using modern theoretical nuclear physics models allows an interpretation of the underlying physical effects.

“Our experimental results and their interpretation with modern theoretical methods show that in the fermium nuclei, nuclear shell effects have a reduced influence on the nuclear charge radii, in contrast to the strong influence on the binding energies of these nuclei,” says Jessica Warbinek, who was a doctoral student at GSI and JGU at the time of the experiments and is first author of the publication. “The results confirm theoretical predictions that local shell effects, which are due to few individual neutrons and protons, lose influence when the nuclear mass increases; instead, effects dominate that are to be attributed to the full ensemble of all nucleons, with the nuclei rather seen as a charged liquid drop.”

The experimental improvements of the method pave the way to further laser spectroscopic studies of heavy elements in the region around and beyond neutron number 152 and represent a step towards a better understanding of stabilization processes in heavy and superheavy elements.stable and thus unreactive, nuclei with filled nuclear shells (containing so-called “magic” numbers of nucleons) exhibit an increased stability. Consequently, their nuclear binding energies and their lifetimes increase.

Support info: heavy elements, quantum mechanical effects and the nuclear shell model

• Elements beyond uranium (element 92) do not occur naturally in the Earth's crust. To be studied, they thus have to be produced artificially.
• Elements like fermium (element 100) form a bridge between the heaviest naturally occurring elements and the so-called superheavy elements.
• Superheavy elements, which start at element 104, owe their existence to stabilizing quantum mechanical shell effects, which add about two thousandths of the total nuclear binding energy; albeit a small contribution, it is decisive in counteracting the disruptive forces acting between the many positively charged protons, which all repel each other.
• Quantum mechanical effects induced by the building blocks of atomic nuclei (protons and neutrons) are explained by the nuclear shell model. Similarly as for atoms, where those with completely filled electron shells are chemically stable and thus unreactive, nuclei with filled nuclear shells (containing so-called “magic” numbers of nucleons) exhibit an increased stability. Consequently, their nuclear binding energies and their lifetimes increase.
• In lighter nuclei, filled nuclear shells are known to also influence trends in the nuclear radii. Indeed, studies of several atomic nuclei of the same element, but with different neutron numbers, have revealed a steady increase in this radius, unless a magic number is crossed. Then, a kink is observed, as the slope of the radial increase changes at the shell closure. This effect was found for lighter, spherical atomic nuclei up to lead.